Chemical vapor infiltration and deposition is a well known technique for depositing a binding matrix within a porous structure. The terminology “chemical vapor deposition (CVD) generally implies deposition of a surface coating, but the terminology is also used to refer to infiltration and deposition of a matrix within a porous structure. As used herein, the terminology “CVI/CVD” refers to infiltration and deposition of a matrix within a porous structure. This technique is suitable for fabricating high temperature structural composites by depositing a carbonaceous or ceramic matrix within a carbonaceous or ceramic porous structure composed of fibers, and is particularly valuable in the manufacture of useful structures such as carbon-carbon composite racing car and aircraft brake discs.
Generally speaking, manufacturing carbon parts using a CVI/CVD process involves placing preformed porous structures in a furnace and introducing a high temperature reactant gas to the porous structures. When carbon-carbon aircraft brake discs are being manufactured, fibrous carbon porous structures typically are treated with a reactant gas mixture of natural gas, often enriched with some propane gas. When the hydrocarbon gas mixture flows around and through the porous structures, a complex set of dehydrogenation, condensation, and polymerization reactions occur, thereby depositing the carbon atoms within the interior and onto the surface of the porous structures. Over time, as more and more of the carbon atoms are deposited onto the structures, the porous structures become more dense. This process is sometimes referred to as densification, because the open spaces in the porous structures are eventually filled with a carbon matrix until generally solid carbon parts are formed.
As is well known to those skilled in the art, critical control variables in chemical vapor infiltration and deposition processes include: preform temperature and pore structure; reactant gas composition, flow rate, temperature, and pressure; and reaction time. The surface reaction of deposition of carbon is an exponential function of the preform temperature. The process therefore is very sensitive to this parameter. Maintaining a controlled uniform temperature throughout the furnace in which preforms are being treated is critical to achievement of consistent densification results.
Based upon these critical control variables, CVI/CVD processes may be broadly classified as:                (1) Conventional—isothermal and isobaric;        (2) Thermal gradient (for example, U.S. Pat. No. 5,348,774); and        (3) Pressure gradient or forced flow (for example, U.S. Pat. No. 5,480,678).        
The present invention relates to the conventional process, which is designed to maintain the preform temperature at a constant (isothermal) with no significant pressure differentials in the furnace (isobaric). In conventional densification, annular brake discs are arranged in stacks with adjacent brake discs stacked on top of one another. A center opening region is thus formed through the center of each stack. As may been seen e.g. in FIG. 2 of U.S. Pat. No. 6,669,988 B2, on the order of a dozen stacks may be located together in a densification furnace. As may be seen e.g. in FIG. 5 of U.S. 2003/0118728 A1, each stack may contain on the order of two score brake disc preforms. Graphite or carbon-carbon spacers are placed between adjacent brake discs to form open passages between the center opening region and the outer region. The reactant gas flows randomly around the stack and may flow through the open passages from the center opening region to the outer opening region or vice versa, with no significant pressure gradients. The stacks may or may not be confined within graphite or carbon-carbon cylindrical structures. Conventional densification treatments are generally conducted for several hundreds of hours.
In conventional processing, the intent is to maintain all of the brake discs at a constant temperature during the process (“isothermal”). This is not, however, successfully achieved in many applications. For instance, the heat source is from the outer diameter of the cylindrical vessel in all of the depictions of the process in U.S. Pat. No. 5,904,957 and U.S. Pat. No. 6,669,988 B2. In such embodiments of the so-called isothermal process, the heat transfer to the stacks located in the center of the furnace is not the same as is the heat transfer to the stacks located near the walls. This results in the center stacks not densifying to the same extent as do the stacks near the walls, which results in the need for additional correction cycles and/or in a potential for undesired microstuctures impacting cost and perhaps variability in friction performance of the brakes.
U.S. Pat. No. 5,904,957 places stacks of annular preforms in a furnace with spacer elements between each of the preforms and between the last preforms in the stacks and the screens at the top end, thus forming leakage passages between adjacent preforms. The gas is then channeled towards only the interior passage of each annular stack at the bottom. The top end of the stacks is closed by solid screens. One disadvantage of this method is that the outer surfaces of the brake disc near the bottom of the stacks may become starved for gas, thereby producing an undesirable densification of the bottom brake discs and non-uniformity between the bottom and top brake discs. Another disadvantage is that the closed top ends of the stacks blocks the gas flow out of the top ends, thus causing gas stagnation problems resulting in seal-coating as well as in undesirable microstructure.
U.S. Pat. Nos. 5,480,678 and 6,109,209 describe an invention said to be particularly suited for the simultaneous CVI/CVD processing of hundreds of aircraft brake discs. This invention includes a gas preheater for use in a CVI/CVD furnace that receives a reactant gas from a gas inlet. A sealed baffle structure is disposed within the furnace. The sealed baffle structure has a baffle structure inlet and a baffle structure outlet. A sealed duct structure is disposed within the furnace, with the sealed duct structure being sealed around the gas inlet and the baffle structure, so that substantially all of the reactant gas received from the gas inlet is directed to and forced to flow through the sealed baffle structure to the baffle structure outlet.
Another method of achieving uniform temperatures in the center stacks is via preheating the reactant gas. This method is described in U.S. 2003/0118728 A1. The reactant gas is typically admitted into the furnace under ambient conditions with a residence time in the furnace on the order of seconds. As is known to those skilled in the art, preheating the gas to near the temperature of the brake discs results in secondary gas phase reactions forming undesirable side products, such as soot and tar, which accumulate on the surfaces of the brake discs and/or on the furnace equipment. Accumulations of soot and tar can cause a number of problems which adversely affect the quality of the brake discs and the cost of manufacturing.
Seal-coating is one typical problem than can result from soot and tar accumulation, although seal-coating can also be caused by other conditions, such as stagnation as described below. The seal-coating results in blocking of surface pores, thus preventing the flow of reactant gas from further infiltrating the occluded porosity in the preform. Surface machining and additional correction cycles become necessary to achieve the required density, increasing cost and reducing uniformity. Accumulations of soot and tar can also make disassembly of intricate close-fitting parts especially difficult, since the tar tends to bind the parts tightly together. The maintenance costs associated with such complex fixtures building up tar over a period of time can be substantial.